The present invention relates to an electrode substrate in which a lower electrode and an upper electrode face each other with an insulating film therebetween and a semiconductor device using the same such as a thin film transistor and a display device, particularly to an electrode substrate in which a lower electrode and an upper electrode whose main shapes are formed as reversed patterns of each other are placed in a self-aligned manner to each other and a semiconductor device such as a thin film transistor and a display device using the above described electrode substrate, and to a method for producing them.
An electrode substrate in which a lower electrode and an upper electrode face each other with an insulating film therebetween includes, for example, the electrode substrate for use in a thin film transistor for an active matrix driving liquid crystal display device. In this electrode substrate, the lower electrode as a gate wiring/electrode, the gate insulating film, and the upper electrode as a source/drain electrode and signal wiring are formed in this order by lamination on a substrate comprised of glass or the like. In order to form a thin film transistor and a display device driven by the same on a substrate having a large area with a high degree of accuracy, the lower electrode and upper electrode for composing wirings/electrodes each need to be processed to form a fine pattern and placed in an accurate alignment with each other. Therefore, so called photolithography method each using separate photomasks is used as a general method to process and form the lower and upper electrodes. In this method, a photomask that is preliminarily processed to form a fine pattern is placed to a (positive type) photoresist deposited on the electrode; the photoresist is subjected to photoirradiation and removed from an exposed region; the electrode is processed by removing the electrode from the uncovered region of the photoresist; and finally the photoresist is removed. The accurate alignment of the photomask to be used for processing each electrode allows each electrode pattern to be accurately aligned.
A back-surface exposure method is known as a method for accurately aligning a lower electrode and an upper electrode. The present invention comprises a method for utilizing “a part” of a lower metal electrode in an auxiliary manner as a photomask for determining “a part” of the pattern shape of an upper electrode. The detail of this method is described, for example, in Japanese Patent No. 3,304,671 by the same inventor.
In recent years, a method for using a printing method that is so called direct-drawing method such as inkjet, plating, and offset printing has been actively studied as a method for forming the electrode for use in these electrode substrates, as described, for example, in Nikkei Electronics, No. 6.17, pp. 67-78, 2002. In these printing methods, necessary materials are placed and formed in necessary locations. Therefore, these printing methods have less number of production processes and higher utilization efficiency of materials than a photolithography process, leading to an expectation to an advantage that an electrode substrate can be formed at a low cost. The above literature introduces an example in which a metal wiring having a line width of 5 μm or less has been formed by an inkjet method, as an example in which a fine electrode pattern has been formed by using a printing method.
A thin film transistor using the electrode substrate described above is utilized in an active matrix driving display device, and used in a flat image-display device using, for example, liquid crystal elements, organic electroluminescent elements, electrophoresis elements and the like as display elements. In addition, there is a move in which a thin film transistor using the above electrode substrate is utilized to RFID, a non-contact information medium, typified by a non-contact IC card. In either case, a thin film transistor is utilized in basic products that support an advanced information-oriented society as a man-machine interface device through the medium of image and communication information.
In the above prior arts, if a photolithography method can be replaced by a printing method as a method for forming an electrode substrate in which a lower electrode and an upper electrode, which have fine pattern shapes and are accurately aligned to each other, face each other with an insulating film therebetween, production processes would be much reduced and utilization efficiency of materials would be improved, leading to an expectation of an advantage that a large number of electrode substrates can be formed at a low cost.
However, it has been difficult to use a printing method, particularly as a method for forming electrodes in the electrode substrate with the above construction due to the following reasons. Specifically, “misregistration” occurs when an electrode with a fine shape, which is formed using a printing method, is transferred onto a substrate from a printing device. This causes the problem that even when at least one of a lower electrode and an upper electrode formed thereon trough an insulating layer can be formed in a fine pattern shape using a printing method, the both cannot be aligned accurately. This will be described with reference to FIG. 12 which shows the problem of misregistration of electrodes in the electrode substrate according to the present invention. FIG. 12 (a) shows a plan view illustrating electrodes that are well aligned and a sectional view taken along line A-A′; and FIGS. 12 (b) to (d) are plan views illustrating electrodes with “misregistration”. A lower electrode 2, an insulating film 3, and upper electrodes 5 and 6 are layered in this order on a substrate 1. In FIG. 12 (a), both sides of the lower electrode 2 match with right end and left end of the upper electrodes 5 and 6 respectively, that is, they are well aligned. On the other hand, FIG. 12 (b) shows an example in which the lower electrode 2 has shifted to the lower right on the substrate surface; FIG. 12 (c) shows an example in which the upper electrodes 5 and 6 have shifted to the upper left on the surface of the insulating film; and FIG. 12 (d) shows an example in which both misregistrations have occurred. The occurrence of misregistration causes loss of the matching of position between the lower electrode 2 and upper electrodes 5 and 6, and causes unnecessary overlapping and separation therebetween, even when electrodes are formed in fine patterns. It is known that, in the case of an inkjet method, such “misregistration” occurs while a conductive ink ejected from a head part is flying until it attaches a substrate, and in the case of a transfer printing method, it occurs when a pattern of a conductive ink is transferred from a transfer roll to a substrate.
This results in a problem that when an electrode substrate of the above construction without the defect of electrode misregistration, it is necessary to use a photolithography method in at least a part of the processes, which prevents the reduction of production processes and improvement of utilization efficiency of materials. Further, there is a problem that when a thin film transistor and a semiconductor device such as a display device using the same are produced using the electrode substrate in which misregistration has occurred as a result of production by a printing method, the performance and uniformity of devices are low and devices cannot be highly integrated with higher definition.
To these problems, it is an object of the present invention to provide an electrode substrate and a method for producing the same in which a lower electrode and an upper electrode, which have fine pattern shapes and are accurately aligned to each other, face each other in self-alignment with an insulating film therebetween, by using a printing method in place of a photolithography method, and to provide a semiconductor device such as a thin film transistor and a display device using the above described electrode substrate and a method for producing the same.